The British actor - who is best known for playing vampire Edward Cullen in the `Twilight Saga` film series - was left perplexed during filming on the first movie when a female supporter handed over her three-month-old child to meet the cast.
He recalled: "My favourite moment from the whole thing was when we doing the first one and there were people waiting by the perimeter fence and this woman handed over her three-month-old baby to the AD [assistant director] to meet us.
"The AD didn`t know what to do with it, so he was like, `I think she wants you to sign the baby or something?` and then we realised she wanted pictures with the baby.
"I didn`t understand the reality of the thing at the time, that people would see the pictures, so there`s a picture of me on the internet somewhere biting this baby."
Despite his revelation, Robert - who is dating his `Twilight Saga` co-star Kristen Stewart - insists he didn`t make physical contact with the infant and only pretended to bite when posing for the picture.
He added: "I didn`t actually, like, touch it. It`s a funny picture. The baby was so small its entire head fit in my mouth."